Apparatus for producing an electron beam



Dec. 9, 1969 c. w. HANKS APPARATUS FOR PRODUCING AN ELECTRON BEAM Filed Dec. 12. 1967 ,5 74 1104 dam megs dnventor C/mzzz'a WAQ/VA/E MAW/14, w

United States Patent US. Cl. 31514 5 Claims ABSTRACT OF THE DISCLOSURE Apparatus is described for producing an electron beam, wherein a reversing transverse magnetic field is established in the path of the electron beam for compensating electrons in the beam for deflection caused by A-C filament heating current.

This invention relates to the production of electron beams and, more particularly, to improved apparatus for producing an electron beam.

Apparatus for producing electron beams may take various forms, depending upon the intended use. Such apparatus typically includes an electron beam gun and associated power supplies. Generally, an electron beam gun includes an emitter or source of electrons, and means for accelerating such electrons into a beam.

In an electron beam furnace, wherein high energy electron beams are utilized to heat metals and other materials in a low pressure environment, an emitter comprising a heated elongated filament is sometimes used. This is a simple construction and facilitates the production of relatively large amounts of electrons for high heating efliciency. The beam produced in an electron beam gun of this type is initially of a long and narrow cross section, and is sometimes referred to as a ribbon beam.

One of the simplest ways of heating 28.11 elongated fila ment in an electron beam gun is by passing an alternating current through the filament and utilizing the resistance of the filament to produce heat. Such a self-heated or directly heated filament is subject to a problem created by the production of an adjacent magnetic field by the heating current. This magnetic field has magnetic lines of force disposed in concentric shells around the filament. The portions of such lines of force lying in the beam path along which the free electrons produced by the filament are accelerated extend transversely of the beam path. Electrons passing through such transverse lines of force are deflected and, since around an A-C heated filament the lines of force are constantly varying in strength and reversing in direction the electrons in the beam are sprayed or swept through a sizeable angle. This angle may, for example, be about 30 with respect to a normal to the filament in a gun operating with a kv. accelerating potential. The angle of sweep may be even greater for lower potentials, since the electron velocity will be lower and the electrons subject to greater deflection by the field. A sprayed or widely diverged electron beam produces generally inefl'icient heating and may be very difficult to control as to direction.

As a result of the foregoing problem, types of electron sources other than an alternating current heated elongated filament have been utilized. The magnetic field at the electron emitting surface has been eliminated in some electron beam gun assemblies by utilizing indirectly heated filaments as sources of electrons. Electron beam gun assemblies utilizing indirectly heated filaments frequently tend to be complex and expensive and, as a result, may be impractical for many purposes.

Another source of electrons which has been utilized is 3,483,423 Patented Dec. 9, 1969 a filament heated by direct current. Where a direct current is utilized to heat an elongated filament, a ribbon beam results which is projected out of the electron beam gun assembly at an angle of, for example, about 10 to 15 with respect to the filament at a 10 kv. accelerating potential. This usually means that the entire gun must be skewed or mounted at an angle with respect to the desired beam path. With some electron beam furnace configurations, a skew mounted gun requires complex structural support and critical adjustment.

Another difiiculty of direct current filament heating arises from the necessity of a high current D-C supply. For electron beam furnace use, the high current DC supply usually must be well filtered both as to voltage ripple in the power supply and as to the reduction of voltage transients, from plasmas generated in the electron beam furnace, moving back into the power supply rectifiers. Such elaborate D-C power supplies may be prohibitively expensive.

In addition to the foregoing difiiculties, many electron beam gun assemblies tend to spray or diverge the electron beam. This tendency is caused at least in part by the fact that the accelerating potentials, which are established on the various elements of the electron beam gun assembly, may fluctuate or ripple. Such fluctuation or ripple may be necessarily present because of the desirability of minimizing the amount of filtering used in the power supply. By minimizing filtering and hence stored energy in the power supply circuitry for the accelerating potentials, operation of the system is safer and arcing may be easier to control.

Ripple voltages on the accelerating elements contribute to beam divergence by causing a spread in electron velocity between those electrons that are accelerated when the accelerating voltage is at a peak and those electrons accelerated when the accelerating voltage is in a valley. The degree of curvature imparted to an electron as it passes through a magnetic field is a function of the strength of the magnetic field and the velocity of the electron. Since the higher velocity electrons are bent less by the field established by the heating current than those electrons of lower velocity, the beam is sprayed. Thus, the diverging effect resulting from variation in the magnetic field produced by the heating current, as discussed above, is often compounded due to voltage ripple on the accelerating elements.

It is an object of this invention to provide improved apparatus for producing an electron beam.

Another object of the invention is to provide apparatus for producing an electron beam having built-in correction for path deviation due to magnetic fields produced by filament heating current.

It is another object of the invention to provide an electron beam apparatus with an elongated filament heated by alternating current, wherein satisfactory beam formation is achieved.

A further object of the invention is to provide electron beam producing apparatus which is low in cost of manufacture and operation, and which is simple of construction.

Other objects of the invention will become apparent to those skilled in the art from the following description taken in connection with the accompanying drawings wherein:

FIGURE 1 is a schematic side elevational view of electron beam apparatus constructed in accordance with the invention;

FIGURE 2 is a schematic top view of the apparatus of FIGURE 1;

FIGURE 3 is a sectional view taken along the line 3-3 of FIGURE 1; and

FIGURE 4 is a diagrammatical perspective view illustrating the operation of the apparatus of the invention.

Very generally, the apparatus of the invention comprises an elongated filament 11 and means 12 for passing an A-C heating current through the filament to produce free electrons. Means 13 are provided for accelerating the free electrons into a beam. Means 14 are provided for establishing a reversing transverse magnetic field disposed in the path of the beam. Such field establishing means include a source 16 of alternating current of the same frequency as the heating current to produce a field of varying strength and direction. The lines of force of the field extend opposite to the effective portions of the lines of force of the magnetic field produced by the heating current. The first named lines of force are of a strength sufiicient to deflect the beam and compensate for initial beam deflection caused by the field Produced by the heating cur rent.

Referring now more particularly to FIGURES 1 through 3, the elongated filament 11 preferably comprises a wire of tungsten or similar material having the characteristic of emitting a large number of free electrons upon heating. The emitter or filament 11 is supported, preferably under tension, between a pair of conductive supports 12. The conductive supports are connected across a source of alternating current, at a power supply 16, such alternating current being passed through the filament 11 for heating same.

The filament 11 extends along a recess or trench 17 in a backing electrode 18, and is spaced from the backing electrode. The backing electrode 18 may be comprised of graphite or similar material and is insulated from one of the alternating current conductive supports 12 by suitable means, not illustrated. The backing electrode 18, being positioned on three sides of the filament 11 helps to shape the electrons emitted by the filament 11 into a ribbon beam.

The means which accelerate the free electrons emitted by the filament 11 into a beam comprise a pair of anode rods 13. The anode rods 13 extend between a pair of conductive supports 19 and are maintained at a positive potential with respect to the filament 11 and with respect to the backing electrode 18 by coupling the backing electrode 18 and the filament 11 to a high negative voltage power supply 21 and by grounding anode rods 13. Accordingly, electrons emitted by the filament 11 tend to flow out of the open end of the recess 17 in the backing electrode and are accelerated into a ribbon-shaped beam by the accelerating anode rods 13.

In order to correct the beam for spraying resulting from the magnetic fields produced by the heating current, means 14 are provided for establishing a correcting magnetic field. The field establishing means 14 in the illustrated embodiment are a pair of elongated pole pieces 14 each constructed of a plurality of strips of transformer iron. The pole pieces 14 are positioned in parallel relation on opposite sides of the electron beam path and just beyond the accelerating anode rods 13. The pole pieces extend between and rest upon a pair of bottom plates 22 of insulating material. The pole pieces are clamped to the bottom plates 22 by clamping plates 23 of insulating mate rial through which clamping screws 24 are passed. One end of each of the pole pieces 14 extends beyond the clamping plate 23 and an electromagnet 26 having an iron core 27 extends between the ends of the pole pieces 14 beyond the clamping plate 23. When the electromagnet 26 is energized by electrical current, a magnetic field is established between the pole pieces 14. If desired, the pole pieces may be made to converge toward the ends thereof opposite the electromagnet 26 in order that the field established between the pole pieces may be of uniform strength along the full length of the gap between the pole pieces. In the illustrated embodiment, the current supply to energize the eiectromagnet 26 is derived from the power supply 16 so that the electromagnet is driven in a predetermined phase relation to that of the filament for a purpose hereinafter described.

Referring to FIGURE 4, it will be noted that the alternating heating current passing through the filament 11 establishes a magnetic field extending around the filament which has lines of force disposed concentrically about the filament. The lines of force, for one direction of current flow, are indicated by the arrows 28. The electrons being accelerated by the anode rods 13 cross the effective portions (those portions in the beam path) of the concentric lines of force 28 generally perpendicular thereto. The electrons are thereby deflected through an angle 0:. Since the magnetic field represented by the flux lines 28 is constantly changing in strength and reversing in direction due to the corresponding sinusoidal characteristic of the heating current, the total amount by which electrons in the electron beam deviate may be considered as twice the angle a.

In order to correct the electrons for their initial deviation resulting from the field produced by the heating current in the filament 11, the transverse field extending between the pole pieces 14 is established. The electromagnet 26 is polarized so that the varying and reversing transverse field between the pole pieces 14 has lines of force which are about equal and opposite to the effective portions of the lines of force 28, The lines of force of the transverse field are represented by the arrows 29 in FIGURE 4. The electrons passing through the transverse field between the pole pieces 14 intersect the lines of force 29 perpendicularly thereof and are deflected through an angle 19. The direction of deflection is generally opposite to that of the initial deflection caused by the lines of force 28. The strength of the magnetic field, as it varies, is selected to compensate for the initial deflection of the electrons in the beam so that the angle ,8 is approximately equal to the angle oz. As a result, the beam produced by the electron beam gun assembly of the invention issues straight out of the gun in a direction substantially perpendicular to the filament 11, and is not widely sprayed. For most purposes, the coil energizing current for the electromagnet 26 may be the identical current as that used for heating the filament 11, thus insuring a proper phase relationship. Under some circumstances, it may be desirable to produce a slight phase difference in the energizing current for the electromagnet 26 in order to compensate for hysteresis in the electromagnet and associated pole pieces.

By utilizing the invention, a beam impact pattern 6 inches in front of the gun assembly may be made only one-fourth as great than would be the case were the correcting pole pieces 14 not utilized.

It has been previously mentioned that direct current high voltage power supplies, used to provide the accelerating voltages on the anodes 13, are usually provided with minimum amounts of filtering. This is because it is desirable to keep the stored energy in the power supply at a low level. Thus, even though such power supplies are usually 3-phase rectified systems, the voltage ripple, peak-to-peak, is relatively large.

As mentioned previously, there is a spread in electron velocity between those electrons that are accelerated when the accelerating voltage is at a peak and those electrons which are accelerated when the accelerating voltage is in a valley. The degree of curvature imparted to an electron as it passes through a magnetic field is a function of the strength of the magnetic field and the velocity of the electron. The higher velocity electrons are bent less than those of lower velocity. As a result of electron velocity spread, electrons coming off of the filament are sprayed by the field produced by the heating current.

By using a correcting magnetic field in accordance with the invention, the effect of varying voltage on the accelerating anodes 13 is automatically compensated for. This compensation occurs because the correcting field imparts a deflection to the electrons which is equal and opposite to their deflection by the heating current field. Thus, those electrons coming out of the filament that are curved an increased amount by the heating current field because of their lower velocity are corrected by the same amount of curvature in the opposite direction by the transverse magnetic field of the correction pole piece system. Such correction is in addition to and coincident with the compensation for field variation from alternating current used to heat the filament.

It may therefore be seen that the invention provides an improved electron beam gun assembly. The necessity of utilizing direct current filament heating or indirect heating as a means of reducing beam spread caused by the heating current is eliminated. Moreover, automatic correction of beam spread caused by differences in the velocity of electrons as accelerating voltage fluctuates also occurs.

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. Apparatus for producing an electron beam comprising, an elongated filament, means for passing an AC heating current through said filament to produce free electrons, means for accelerating said free electrons into a beam, and means for establishing a reversing transverse magnetic field in the path of the beam, said magnetic field establishing means including a source of alternating current of the same frequency as the heating current to produce a field of varying strength and direction having lines of force extending opposite to the effective portions of the lines of force of the magnetic field produced by the heating current, said first named lines of force being of a strength to deflect the beam and compensate for initial beam deflection caused by the field produced by the heating current.

2. Apparatus according to claim 1 wherein said magnetic field establishing means are adapted to produce a field of varying strength and direction substantially in phase with the varying field produced by the heating current.

3. An electron beam gun assembly according to claim 1 wherein said magnetic field establishing means include .a pair of elongated pole pieces positioned on opposite sides of the electron beam path, and further include an electromagnet extending between said pol-e pieces proximate one end of said pole pieces.

4. An electron beam gun assembly according to claim 3 wherein each of said pole pieces is comprised of a plurality of strips of magnetic material.

5. An electron beam gun assembly according to claim 3 wherein said pole pieces are positioned to converge toward the ends thereof opposite said electromagnet to establish a uniform transverse field.

References Cited UNITED STATES PATENTS 3,172,007 3/1965 Hanks et al 31514 JAMES W. LAWRENCE, Primary Examiner RAYMOND F. HOSSFELD, Assistant Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NU. 3,483,423 Dated December 9, 1969 Invent0r(s) Charles W. Hanks It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 9, "on" should be --5.

illifiiii AWL QFALED uav 10 (SEAL) Attest:

Edwardm'memj' mm B- saaun-m, .m. Attesting Officer comissioner of Patents 

